Dico wrote:
Hello all,
I was looking through the TSIO-550 Power Settings Chart and noticed that it said the min MP for pressurization was 31.5".... I'm just wondering if this is true for all IVPs?
I was up to FL170 the other day at 28.5" and the cabin was showing perhaps 7800. Should I assume that if I was running at 31.5" that the cabin might only have been showing perhaps 5000'? The reason I ask is because I'm thinking if we're at FL170 and the
cabin is pushing 8000, then at FL250 (or even FL210) the cabin pressure would be well over 10,000'. I had thought it might have been because I have a leaky cabin or something.... but perhaps its because I'm being too cheap with the fuel....
Dico,
The Lancair IV-P is designed to operate at a maximum differential pressure (pressure difference between the cabin interior
and outside) of 5 PSI. Your airplane has a Duke’s pressurization/control system that allows you to set the cabin altitude to a specified value. Most are calibrated to generate a cabin altitude of 1,000-1,500’ above the set altitude—for a couple of reasons.
In any case, the system will allow pressurized air from the turbochargers to flow through the cabin until the inside pressure altitude reaches about 1,000’ above what’s set on the control. It then starts closing the outflow valve under the rear seat to maintain
this inside pressure altitude. As long as there is a sufficient supply of pressurized air from the
turbos, the controller will automatically close/modulate as needed to maintain this altitude-- until the inside/outside difference reaches 5 PSI. The cabin pressure then starts to decrease (higher cabin altitude) as you continue
to climb. I’ve attached a chart that shows that with a 5 PSI differential system, you should be able to maintain an 8,000’ cabin altitude to about 23,000’ PA and that the cabin should be about 10,000’ at FL 250. Because of leakage, inefficiencies and calibration
settings , the cabin altitude starts to go up as the airplane climbs through FL 21-220---for most of us.
There are two big factors that govern how well this works—supply pressure/volume and cabin leakage. A high engine power
setting generates a lot of high-pressure air from the turbos. This is moderated somewhat by the sonic nozzles in the hot side feed (some airplanes were built with sonic nozzles in both the hot and cold feed lines; some did not do
this), but you can definitely tell a difference in airflow through the cabin between takeoff and cruise power settings.
If the cabin is leaky, even max power (38.5” boost pressure) won’t generate enough high pressure air to pressurize the
cabin (e.g., you forgot to activate the door seal). If it’s a tight cabin, 25” of MP is about the minimum that will maintain the 5 PSI difference…and since there are not anti-back flow valves in this system, pulling the power below this will cause pressurized
cabin air to flow back through the engine—rather rapidly.
To answer your specific question, you should be able to maintain a cabin altitude of about 4500’ IF your altitude controller
was set below 3,000’ AND your cabin is tight. You can easily check for a calibration or leakage problem by stabilizing at 17,500’ in cruise with the altitude controller set to 0—or at least below 3,000’--and check the cabin pressure. Then increase MP by at
least 2” of Hg and again check the cabin pressure. If it increases (cabin altitude decreases), you can improve the cabin pressure by sealing leaks; if it doesn’t change, you can improve the cabin pressure by adjusting the pressure controller.
If you can’t maintain cabin pressure at 25-26” of MP, consider working on the cabin sealing. Requiring 31” of MP to maintain
cabin pressure indicates leaks to me.
Bob